NullReferenceException happened when a server replied with an error to an auth info request

this usually happens when you connect to a server that requires encryption but you have not enabled tls
Merge pull request #12 from oli-obk/fix/nullref
Merge branch 'stable' into master

Conflicts:
	agsxmpp/XmppClientConnection.cs
Merge pull request #14 from oli-obk/merge/pr#12